Oxide film coating on silicon in a semiconductor manufacturing process requires 1,000 sccm (1,000 cc/minute under standard conditions) of high-purity moisture.
For that purpose and others, the present inventors had earlier developed a reactor for generation of moisture of a construction as shown in FIG. 6 and disclosed the same in Japanese patent application No. 10-297907.
In FIG. 6, the reference character A indicates a reactor shell; the numeral 1, a reactor structural component on an inlet side; the numeral 1a, a gas fed port; the numeral 2, a reactor structural component on an outlet side; the numeral 2a, a moisture gas take-out port; the numeral 3, an interior space on the inlet side; the numeral 4, an interior space on the outlet side; the numeral 5, a reflector on the inlet side; the numeral 6, a reflector on the outlet side; the numeral 7, a metal filter; and the numeral 8, a platinum coat catalyst layer 8.
The platinum coat catalyst layer 8 is provided on the inside surface of the reactor structural component 2 on the outlet side. The layer is formed of a barrier coat 8a, for instance, of TiN on the inside wall surface of the reactor structural component 2 on the outlet side and a platinum coat 8b fixed thereon.
To produce moisture, a mixed gas G prepared by mixing H2 and O2 at a specific ratio is fed into the reactor shell A. The mixed gas G fed into the interior space 3 on the inlet side of the reactor shell A is diffused by the reflector 5 on the inlet side and the metal filter 7, and flows into the interior space 4 on the outlet side. There, the mixed gas G comes in contact with the platinum coat 8b, which activates O2 and H2.
H2 and O2, which are activated in contact with the platinum coat 8b, react into moisture gas (water vapor) at a temperature of as high as about 300 to 500° C. The moisture gas (water vapor) thus produced flows out to the moisture gas take-out port 2a through the gap L between the reflector 6 and the reactor structural component 2 on the outlet side. From the moisture gas take-out port 2a, the moisture gas is then supplied to a process chamber (not shown) for semiconductor manufacturing facilities.
In the reactor shell A, where O2 and H2 react at a high temperature, the temperature in the interior spaces 3, 4 is maintained at a temperature below the ignition point of H2 or an H2-containing gas so that H2 and O2 are reacted at a proper rate to produce moisture gas at a specific flow rate while H2 and O2 are prevented from causing an explosive reaction.
The reactor shell A shown in FIG. 6 is very small in size but capable of producing high-purity moisture continuously at a desired flow rate and at a high reaction rate in a simple procedure. Thus, the reactor shell A is very practical and useful.
However, the reactor shell A shown in FIG. 6 has a number of problems yet to be solved. Among them, the problems requiring urgent solution are (1) ignition of H2, and backfire from the gas feed port 1a to the gas supply source, and (2) partial peeling off and coming off of the platinum coat catalyst layer 8 owing to a local sudden rise in temperature in the reactor shell A.
As mentioned above, the temperature in the interior spaces of the reactor shell A is maintained at about 450 to 500° C.—the temperatures much lower than the lowest ignition point of H2 or an H2-containing gas. The lowest ignition point is about 560° C. and varies to some degree depending on the mixing ratio of H2 and O2. Thus, H2 and O2 are prevented from undergoing a sudden, explosive combustion reaction.
In practice, however, it is very difficult to keep the temperature below the lowest ignition point perfectly and continuously in the interior spaces 3, 4 of the reactor shell A. It can happen that the temperature rises above the lowest ignition point locally on the inside wall of the reactor structural component 1 on the inlet side, the reactor structural component 2 on the outlet side or the like for some reasons.
Even if the temperature rises above the lowest ignition point locally on the inside wall of the reactor structural component 1 on the inlet side or the reactor structural component 2 on the outlet side, O2 and H2 will not always undergo an explosive combustion reaction, causing a backfire toward the gas supply source. Usually, there is no ignition or backfiring. However, it can happen in rare cases that H2 will be ignited or backfire to the gas supply source, especially if H2 is present in a high concentration in the mixed gas G.
It is still not yet known what causes the temperature to rise locally and suddenly in the reactor structural components 1, 2, the metal filter 7 or the like, igniting H2 or triggering a backfire.
The primary cause for ignition of H2 is considered to be that H2 and O2 in the mixed gas G are activated by metallic catalytic action on the inside wall surface of the reactor structural component 1 on the inlet side, the outer surfaces of such other component parts of the reactor shell A as the reflector 5 on the inlet side, reflector 6 on the outlet side, metal filter 7 and the like, causing the temperature to rise too high locally and suddenly at the aforesaid inside wall or the like. This observation is based on the inventors' past experience in building and applying reactors for generation of moisture.
Such parts as the reactor structural component 1 on the inlet side, reflectors 5, 6 and metal filter 7 are all made of stainless steel under JIS designation SUS316L. The outer surfaces of those parts are covered with oxide film and passive state film of a variety of metals which are usually formed naturally. Those films restrict what is called catalytic reactivity that is usually observed on the surface of stainless steel.
If the aforesaid oxide film and passive state film are exposed to the mixed gas G containing H2 in a high concentration at a temperature as high as 450 to 500° C. for a long time, the oxide film and the like can peel off or come off in some cases. In other cases, the oxide film can be reduced. As a result, the metal surface is locally exposed and bare. In addition, the metal catalytic activity on the outer surface of stainless steel is put to work, intensively accelerating a local reaction between O2 and H2. That will raise the temperature at local areas other than the portion provided with the platinum coat catalyst layer 8 in the interior spaces 3, 4 of the reactor shell A. Also, the temperature will rise above the lowest ignition point of H2 or an H2-containing gas.
Meanwhile, it is known that the temperature on the inside wall provided with the platinum coat catalyst layer 8 in the reactor structural component 2 on the outlet side rises high especially in the center of the reactor structural component 2 on the outlet side. Especially when the mixed gas G as used is diluted with N2 to increase the flowing velocity and flow rate, the temperature will further rise in the portion from the periphery to the center of the reflector 6 on the outlet side.
If, therefore, the cause of the ignition and backfire to the gas supply source side lies in the inside wall provided with the platinum coat catalyst layer 8 of the reactor structural component 2 on the outlet side, the following theory may be set up. That is, with a sudden increase in the quantity of the mixed gas G flowing in the gap L, the reaction between H2 and O2 will be further activated in the part of the platinum coat catalyst layer 8 opposite to the peripheral edge of the reflector 6 on the outlet side. As a result, the temperature on the inside wall will suddenly rise to reach the lowest ignition point and ignite H2 or cause the platinum coat catalyst layer 8 to partially come off.
One way that could be thought of to prevent the temperature from locally rising excessively in the interior spaces 3, 4 of the reactor shell A is to enlarge the reactor shell A to increase the thermal capacity and to provide a heat dissipation or cooling unit so as to raise the cooling capacity.
Yet, semiconductor manufacturing facilities are typically installed in a clean room and usually a large space for their installation is not available. Therefore, size reduction is an especially important requirement imposed on the reactor for generation of moisture auxiliary to the semiconductor manufacturing facilities. It is practically impossible to adopt the method by which the platinum coat catalyst layer 8 is prevented from the peeling off owing to local and sudden rise in temperature in the reactor for generation of moisture as mentioned above by enlarging the reactor shell A and providing a cooling unit.
The present invention addresses those problems with the prior art reactor for generation of moisture such that even if the temperature in the inside space of the reactor structural component 1 on the inlet side and the reactor structural component 2 on the outlet side of the reactor shell A is kept substantially lower than the lowest ignition point of H2 or H2-containing gas, H2 is sometimes ignited, backfire to the gas supply source side occurs, or the platinum coat catalyst layer 8 will partially come off while moisture is being generated using the mixed gas with H2 in a high concentration.